1. Field of the Invention
The present invention relates to a television (TV) link infrared (IR) demodulation circuit applied to a TV receiver such as set-top box, and more particularly, to a TV link infrared (IR) demodulation circuit in a set-top box connected to a plurality of TVs, the TV link IR demodulation circuit improving control sensitivity for remote-controlling TVs connected to one set-top box for each multi-room by more accurately detecting a weak signal from a TV link to remotely control a TV.
2. Description of the Related Art
In general, TV receivers such as set-top boxes receive and transmit a plurality of broadcasts such as terrestrial broadcasts and satellite broadcasts to a plurality of TV sets.
FIG. 1 is a functional diagram illustrating a conventional IR demodulation circuit 10 in a set-top box.
Referring to FIG. 1, the set-top box 10 receiving terrestrial broadcasts and satellite broadcasts includes a radio frequency (RF) modulator 11, a divider 12, an infrared (IR) demodulation circuit 13, and a main controller 14. A TV1 20 at a close range and a TV2 30 at a distant range may be connected via the divider 12.
In this case, for example, the TV1 20 at a close range is closely connected to the set-top box 10 installed in the same space such as a living room or a main room. In this case, the TV may be directly controlled by a remote control since the set-top box 10 processing remote control is adjacently located.
For example, the TV2 30 at a distant range is installed in a different space from the set-top box 10, such as a basement or an attic. The TV2 30 at a distant range is distantly connected to the set-top box 10. In this case, since the set-top box 10 processing remote control is located in a distant place in which remote control is difficult, an amplitude of a signal from the remote control is modulated into 7.2 MHz by a TV link 40 and transmitted to the set-top box 10 located in a distant range.
In this case, the remote control signal is demodulated by the IR demodulation circuit 13 included in the set-top box 10 and transmitted to the main controller 14. Accordingly, the main controller 14 may perform a control corresponding to the operation of the remote control, such as controlling a change of a modulated channel of the RF modulator 11.
On the other hand, the TV link 40 receives an IR signal from the remote control, modulates an amplitude of the IR signal into a carrier frequency such as 7.2 MHz included in a frequency range from 7 to 8 MHz, and transmits a remote control signal to the IR demodulation circuit 13 of the set-top box 10. In this case, when a level of the remote control signal is high, a harmonic of 7.2 MHz may interfere with a TV signal. Accordingly, the remote control signal may have a low level as possible and the IR demodulation circuit 13 has to more accurately detect a weak remote control signal with a low level such as 0.380 V regardless of a temperature change.
FIG. 2 is a configuration diagram illustrating a conventional IR demodulation circuit.
Referring to FIG. 2, the IR demodulation circuit includes a transistor Q1 having a base connected to an input terminal In via an input coupling capacitor Cin, a bias resistor R1 connected between the base and an operating voltage Vcc and a bias resistor R3 connected between a collector of the transistor Q1 and the operating voltage Vcc, and a bias resistor R2 connected between the base and a ground and a bias resistor R4 connected between an emitter of the transistor Q1 and the ground. An output terminal Out is connected to the collector of the transistor Q1 via an output coupling capacitor Cout.
Considering operations of the IR demodulation circuit as described above, as shown in FIG. 3, a remote control signal inputted via the input terminal In includes control information and the transistor Q1 is turned on and outputs a low level in a section in which the remote control signal has a high level and is turned off and outputs a high level in a section in which the remote control signal has a low level, thereby demodulating the remote control signal into a pulse signal including a high level and a low level.
On the other hand, since the conventional IR demodulation circuit has a transistor whose operating characteristics varies with a temperature change, an operating range in which a remote control signal with a relatively low level cannot normally be detected as described above.
This will be described referring to FIG. 3.
FIG. 3A is a time chart illustrating a main voltage waveform of the IR demodulation circuit of FIG. 2 when a base-emitter voltage Vbe is set as 0.5 and a turn-on voltage Von of a transistor Q1 is 0.65 Von at a temperature of 25° C. and is set as 0.4 and a turn-on voltage Von of the transistor Q1 is 0.45 V at a temperature of 125° C., respectively.
Referring to FIG. 3A, when a temperature of the transistor Q1 is 25° C., since the base-emitter voltage Vbe is 0.5 V, the turn-on voltage Von is 0.65 V, and an input signal Vin of 0.380 V is inputted while being coupled with the base-emitter voltage Vbe of 0.5 V, a maximum base voltage (Vbe+Vin/2) is 0.69 V when the input signal Vin is coupled with the base-emitter voltage Vbe. Accordingly, the transistor Q1 is turned on when the maximum base voltage (Vbe+Vin/2) of 0.69 V is higher than the turn-on voltage Von of 0.65 V and is turned off since the base voltage (Vbe=0.5 V, Vin=0) is lower than the turn-on voltage Von of 0.65 V when there is no input signal. According to the operations of the transistor Q1, an output voltage Vout is outputted in the form of a pulse signal with a low level and high level.
Referring to FIG. 3A, when the temperature of the transistor Q1 is 125° C., since the base-emitter voltage Vbe is 0.5 V, the turn-on voltage Von is 0.45 V (0.65−(2 mV/Δ1° C.)), and the input signal Vin of 0.380 V is inputted while being coupled with the base-emitter voltage Vbe of 0.5 V, a maximum base voltage (Vbe+Vin/2) is 0.69 V when the input signal Vin is coupled with the base-emitter voltage Vbe. Accordingly, the transistor Q1 is turned on when the maximum base voltage (Vbe+Vin/2) of 0.69 V is higher than the turn-on voltage Von and is turned on since the base voltage (Vbe=0.5 V,Vin=0) is higher than the turn-on voltage of 0.45 when there is no input signal. In this case, since the transistor Q1 is continuously turned on regardless of whether there is an input signal, the output voltage Vout continuously has a low level and it is impossible to normally detect a remote control signal.
To solve the problem described above, in the case where the base-emitter voltage Vbe of the transistor Q1 is set as 0.4 V will be described referring to FIG. 3B.
Referring to FIG. 3B, when a temperature of the transistor Q1 is 25° C., since the base-emitter voltage Vbe is 0.4 V, the turn-on voltage Von is 0.65 V, and an input signal Vin of 0.380 V is inputted while being coupled with the base-emitter voltage Vbe of 0.4 V, a maximum base voltage (Vbe+Vin/2) is 0.59 V when the input signal Vin is coupled with the base-emitter voltage Vbe. Accordingly, the transistor Q1 is turned off when the maximum base voltage (Vbe+Vin/2) of 0.59 V is lower than the turn-on voltage Von of 0.65 V and is turned off since the base voltage (Vbe=0.4 V, Vin=0) is lower than the turn-on voltage Von of 0.65 V when there is no input signal. In this case, since the transistor Q1 is continuously turned off regardless of whether there is an input signal, the output voltage Vout continuously has a high level and it is impossible to normally detect a remote control signal.
Referring to FIG. 3B, when the temperature of the transistor Q1 is 125° C., since the base-emitter voltage Vbe is 0.4 V, the turn-on voltage Von is 0.45 V (0.65−(2 mV/Δ1° C.)), and the input signal Vin of 0.380 V is inputted while being coupled with the base-emitter voltage Vbe of 0.4 V, a maximum base voltage (Vbe+Vin/2) is 0.59 V when the input signal Vin is coupled with the base-emitter voltage Vbe. Accordingly, the transistor Q1 is turned on since the maximum base voltage (Vbe+Vin/2) of 0.59 V is higher than the turn-on voltage Von of 0.4 V and is turned on since the base voltage (Vbe=0.4 V,Vin=0) is lower than the turn-on voltage of 0.45 when there is no input signal. According to the operations of the transistor Q1, the output voltage Vout is outputted in the form of a pulse signal with a low level and high level.